1. Field of the Invention
The present invention relates to a ferrite magnetic material for magnetic recording and a process for the preparation thereof, and more particularly relates to a finely divided ferrite magnetic material, which is suitable for use in perpendicular magnetic recording system, and to a process for the preparation of the magnetic material.
2. Description of Prior Arts
Heretofore, there has been used a magnetic recording system wherein a recording medium such as magnetic recording tape is magnetized along its longitudinal direction in the surface area thereof. A perpendicular magnetic recording system has been recently proposed for accomplishing high density magnetic recording, a various magnetic mediums for use in this system are under study.
There are known methods wherein a magnetic material layer is formed on a substrate (i.e., support) such as a film by a sputtering process, a vacuum deposition process, etc. as the method of preparing the magnetic recording medium for use in the perpendicular magnetic recording system. For example, there has been developed a magnetic recording medium wherein a layer of a magnetic material such as cobalt-chromium is formed on a substrate by the sputtering process.
However, the process for preparing the magnetic recording medium utilizing said sputtering process or vacuum deposition process has disadvantages in productivity and product quality as compared with the conventional process for preparing the magnetic recording medium utilizing the conventional coating process. Accordinagly, methods utilizing the coating process are also studied as the process for preparing the magnetic recording medium for use in the perpendicular magnetic recording system.
For example, there has been proposed a process for preparing the magnetic recording medium for use in the perpendicular magnetic recording system wherein a hexagonal ferrite such as hexagonal barium ferrite in the form of fine particles having a hexagonal plate shape is used as the magnetic material, the hexagonal ferrite is mixed with and dispersed in a resin (binder) and the surface of a substrate is coated with the ferrite.
As said hexagonal ferrite in the form of fine particles having a hexagonal plate shape, magnetoplumbite type hexagonal crystal ferrite is known and represented by the following formula: EQU RFe.sub.12-2x M.sub.x M'.sub.x O.sub.19
wherein R is at least one metal atom selected from the group consisting of barium, strontium and lead, M is at least one divalent metal atom selected from the group consisting of cobalt, nickel and zinc, M' is at least one tetravalent metal atom selected from the group consisting of titanium, zirconium and hafnium, and x is a number ranging from 0.6 to 1.0.
As typical processes for preparing the hexagonal ferrite such as hexagonal barium ferrite used as the magnetic material for the above-mentioned magnetic recording medium, there are known wet processes (such as co-precipitation process and hydrothermal synthetic process), vitrification process, etc.
Among the above-mentioned processes, the preparation of the hexagonal ferrite magnetic material by the vitrification process is generally carried out in the following manner. A mixture of starting materials containing the desired ferrite component and a glass-forming component is melted and then rapidly cooled to form an amorphous material which is then heat-treated as such to form and deposit hexagonal ferrite crystals therefrom, and other materials such as glass component, etc. than the ferrite crystals are removed from the material obtained from the above heat-treatment (hereinafter referred to as heat-treated material).
The starting material mixture for used in the preparation of the hexagonal ferrite magnetic material by the vitrification process usually contains a basic component for the hexagonal ferrite, a coercive force-reducing component and a glass-forming component.
As the basic component for the hexagonal ferrite, Fe.sub.2 O.sub.3 in combination with a metal oxide or oxides such as BaO, SrO and PbO is used. As the coercive force-reducing component, a combination of a divalent metal oxide or oxides such as CoO, NiO and ZnO with a tetravalent metal oxide or oxides such as TiO.sub.2, ZrO.sub.2 and HfO.sub.2 is used. As the glass-forming component, boron oxide (B.sub.2 O.sub.3) is generally used. Alternatively, there may be incorporated in the starting material mixture the above-mentioned components in the form of compounds or salts (for example, carbonate, nitrate and boric acid for boron) capable of being converted into the above-mentioned oxides under heating conditions in the melting range of the starting material mixing.
The ferrite magnetic material in the shape of hexagonal plate, which is suitable for use in the perpendicular magnetic recording system, is in the form of fine particle wherein the diameter of the hexagonal plate is 0.1 .mu.m or below and the thickness thereof is 0.03 .mu.m or below. In order to prepare such magnetic material in the form of fine particle, there is used in the conventional vitrification process, for example, a process wherein the molten starting material mixture containing the above-mentioned components is quenched to convert it into an amorphous material which is then heat-treated to form and deposit a hexagonal ferrite crystal. As the method of quenching the molten mixture, there is used a method wherein the molten starting material mixture is brought into contact with the surface of rotating metallic rolls by pouring the molten mixture onto said surface (called roll method).
Although the ferrite crystal is formed and deposited by the heat treatment of the amorphous material in the process mentioned above, the formation of the nucleus of the ferrite crystal partly appears in the quenching stage of the molten mixture. However, the quenched material is essentially composed of an amorphous material, so that it is called amorphous material as a whole.
It is desirable that the breadth of particle size distribution of the fine particles of the hexagonal ferrite magnetic material used for magnetic recording material is as narrow as possible, and further that any of coarse particles and particles having an extremely small particle size is not contained. This is because noise is caused by the coarse particle in the magnetic recording medium and the presence of the extremely small particle makes the magnetic recording medium magnetically unstable. Thus, it is desirable that the breadth of particle size distribution of the hexagonal ferrite magnetic material used for magnetic recording medium is as narrow as possible, but the breadth of particle size distribution of the magnetic material obtained by conventional vitrification process is wide and relatively large amounts of coarse particles and extremely small particles are contained. Therefore, it is not said that conventional vitrification process is practically excellent process for preparing the hexagonal ferrite magnetic material. Accordingly, it is highly desired to develop a process for preparing a hexagonal ferrite magnetic material, said process being capable of obtaining a ferrite magnetic material in which the breadth of particle size distribution is narrower than that obtained by conventional vitrification process.